One of the important video transcoding applications is bit rate reduction transcoding. Rate reduction transcoding is typically achieved using either truncation based transcoding or by requantization-based transcoding. Selective coefficient truncating transcoding typically achieves a relatively small amount of bit rate reduction and is rarely used often in typical video transcoding applications.
Requantization-based transcoding achieves rate reduction by requantization using coarser quantization steps. Since the transform-domain coefficients are coarsely quantized, blocking artifacts may be produced. In particular, blocking artifacts arise as a result of each block being compressed independently. Blocking artifacts may appear as edges or produce ringing between blocks.
Numerous methods have been developed to reduce the blocking artifacts for decoded pixel blocks. Specifically, current deblocking applications typically decode the pixel blocks before deblocking and re-encode the pixel blocks after decoding. However, for transcoding applications, the decoding and re-encoding cycle may result in increased blocking artifacts, as the decoding and re-encoding may rely on information that is not be available. Furthermore, due to the higher compression provided by transcoding applications, blocking artifacts get worse as the compression is increased. For example, requantization-based transcoding applies the requantization on the transform-domain coefficients so that a costly inverse transformation is avoided. Currently, deblocking cannot be performed in conjunction with transcoding in the transform domain.